A sequence of magmatic and metasedimentary rocks from the Susten pass area (Aar Massif, Central Alps of Switzerland) provides evidence of an orogenic cycle of Ordovician age overprinted by the Variscan orogeny. Several stages of this evolution have been dated by U-Pb on zircon, titanite and monazite, complementing previously published U-Ph zircon data from the same area. A gabbro, with epsilonNd values of +4 to +7 was formed at 478 +/- 5 Ma and was then affected by a HP metamorphic stage. A subsequent HT metamorphic event was succeeded by partial melting during decompression at about 450 Ma.Titanite in calcsilicate marble and monazite in metapelite formed at 319 +/- 3 Ma and 317.5 +/- 2 Ma, respectively. This Variscan metamorphism probably reached amphibolite facies conditions and considerably changed the whole-rock chemical and isotopic compositions of the amphibolites, but it left the U-Pb systems in zircon virtually unaffected. The association of mafic and ultramafic rocks together with sediments could be generated in both convergent and divergent settings. The metamorphic overprints blurred the original geochemical and isotopic characteristics too much to permit a specific geodynamic interpretation. The studied rock association is suggested to be part of an ancient active or passive margin sequence of the peri-Gondwanan Hun terrane.

The pre-Mesozoic metamorphic pattern of the External Massifs, composed of subunits of different metamorphic histories, resulted from the telescoping of Variscan, Ordovician and older metamorphic and structural textures and formations. During an early period, the future External Massifs were part of a peri-Gondwanian microplate evolving as an active margin. Precambrian to lower Palaeozoic igneous and sedimentary protoliths were reworked during an Ordovician subduction cycle (eclogites, granulites) preceding Ordovician anatexis and intrusion of Ordovician granitoids. Little is known about the time period when the microcontinent containing the future External Massifs followed a migration path leading to collision with Laurussia. Corresponding rock-series have not been identified. This might be because they have been eroded or transformed by migmatisation or because they remain hidden in the monocyclic areas.Besides the transformations which originated during the Ordovician subduction cycle, strong metamorphic transformations resulted from Variscan collision when many areas underwent amphibolite facies transformations and migmatisation. The different subunits composing the External Massifs and their corresponding P-T evolution are the expression of different levels in a nappe pile, which may have formed before Visean erosion and cooling. The presence of durbachitic magmatic rocks may be the expression of a large scale Early Variscan upwelling line which formed after Variscan lithospheric subduction. Late Variscan wrench fault tectonics and crustal thinning accompanied by high thermal gradients triggered several pulses of granite intrusions.

The Gotthard massif shows a coherent lithostratigraphy and can be subdivided into units, which can be traced across the entire massif. Indications of possible protholiths, relics of metamorphic parageneses, and magmatic events permit to classify the units chronologically and to distinguish major orogenic cycles. We propose the following revised lithostratigraphic subdivision of the Gotthard massif: ― Uppermost Carboniferous, Permian and Mesozoic sedimentary covers (with some outcrops of Permo-Carboniferous volcanic rocks) ― Late Variscan granitoids (divided into two cycIes by a deformation phase) ― Middle Paleozoic metasedimentary rocks ― Late Ordovician metagranitoids ― Proto Gotthard (pre-Late Ordovician): ― Migmatitic gneisses ― Metagabbros, with island arc affinity ― Metabasalts, metagabbros and meta-ultramafics, with ophiolitic affinity ― Metasedimentary gneisses

The Gotthard massif shows a coherent lithostratigraphy and can be subdivided into units. which can be traced across the entire massif. Indications of possible protholiths, relics of metamorphic parageneses, and magmatic events permit to classify the units chronologically and to distinguish major orogenic cycles. We propose the following revised lithostratigraphic subdivision of the Gotthard massif: Uppermost Carboniferous, Permian and Mesozoic sedimentary covers (with some outcrops of Permo-Carboniferous volcanic rocks) Late Variscan granitoids (divided into two cycles by a deformation phase) Middle Paleozoic metasedimentary rocks Late Ordovician metagranitoids Proto Gotthard (pre-Late Ordovician): Migmatitic gneisses Metagabbros, with island arc affinity Metabasalts, metagabbros and meta-ultramafics, with ophiolitic affinity Metasedimentary gneisses

Mineral assemblages from a large gabbro body in the Kastelhorn area (Gotthard massif, central Switzerland) are described. Numerous relic textures and occasional relic compositions are found. The textures are compared to textures from mafic rocks in the Gotthard massif where relic mineral compositions allow the derivation of the P-T-t path. Typical textures are symplectites and coronites. They show the replacement of primary magmatic assemblages by high-P minerals and subsequent assemblages of granulite and amphibolite facies. Retrograde overprint at greenschist facies conditions is generally weak. Relic minerals indicating an eclogite facies event are symplectites with sodic clinopyroxene + plagioclase and kyanite inclusions in garnet and plagioclase. The dominant amphibolite facies mineral assemblage is hornblende + plagioclase + garnet +/- quartz +/- biotite.

While the evolution of the Mesozoic rocks belonging to the Helvetic realm in the Central Alps has been investigated in detail with respect to deposition, metamorphism, and deformation, much less is known about the pre-Mesozoic history of the basement rocks in the Helvetic domain. The present study is an attempt to summarize the petrography and geology of the pre-Variscan portions of the Aar massif. Often used subdivisions are critically discussed. The importance of considering the Variscan and Alpine retrograde alteration for any petro-tectonic subdivision of the Aar massif is demonstrated. The following main geologic units with similar lithologies and metamorphic evolution are distinguished: (1) The migmatitic Innertkirchen-Lauterbrunnen Crystalline Zone (ILC). (2) a zone comprising the Erstfeld Gneiss Zone (EGZ) with high-grade to anatectic metasedimentary rocks and minor orthogneisses, the Guttannen Unit (GU) consisting of a layered series of metasediments with strong retrogression, and the Ofenhorn-Stampfhorn Unit (OSU) with amphibolite-facies to anatectic metasediments and major mafic and ultramafic inclusions. (3) The Southern Gneiss Zone (SGZ) with high-grade to anatectic quartzofeldspatic and mafic rocks of igneous as well as of sedimentary origin. This subdivision is primarily based on the tectonic and metamorphic situation in the Western and Central Aar massif. However, lithologic units of the Eastern Aar massif can be attributed to the main units as outlined before. Recent studies indicate a Caledonian age for the amphibolite-facies metamorphism and subsequent anatectic event in the Innertkirchen-Lauterbrunnen Crystalline Zone and the more internal units. No age data are available for the Southern Gneiss Zone but a similar metamorphic evolution is likely despite a probable different paleotectonic origin.